10.9.1 Declare and Define Static Members

When a class has static data members, it is not enough to declare
the static member; you must also define it. For example:

class Foo
{
...
void method();
static int bar;
};

This declaration only establishes that the class Foo has an
int named Foo::bar, and a member function named
Foo::method. But you still need to define bothmethod and bar elsewhere. According to the ISO
standard, you must supply an initializer in one (and only one) source
file, such as:

int Foo::bar = 0;

Other C++ compilers may not correctly implement the standard behavior.
As a result, when you switch to g++ from one of these compilers,
you may discover that a program that appeared to work correctly in fact
does not conform to the standard: g++ reports as undefined
symbols any static data members that lack definitions.